Rotary type actuators are commonly used for positioning magnetic read/write heads on recording tracks of a magnetic disk. Exact positioning of the actuator to ensure proper tracking of the magnetic read/write heads is of paramount concern in producing a quality disk drive. As track densities increase, the construction of the actuator must be capable of providing consistent and reliable positioning. In many disk drive systems, the construction of an actuator assembly may include multiple actuator arms and multiple suspensions which position corresponding multiple read/write heads on a disk pack having multiple magnetic disks. Because of the increased size of such an actuator assembly, one continuing concern in the design of such an assembly are the vibrational characteristics of the actuator assembly. The actuator assembly should be of a design that limits undesirable actuator vibration. Providing the requisite stiffness in an actuator assembly is often difficult because of a number of design limitations to include the inability to integrally mold or extrude each component of the actuator. For example, a voice coil must be attached to the actuator assembly, typically by an over-molded thermoplastic resin. Because the resin and voice coil cannot be integrally molded or extruded with the body of the actuator assembly, complex vibrational variables are introduced in operation of the actuator assembly. Thus, one inherent problem in use of any type of thermoplastic resin to attach the voice coil is that the thermoplastic resin itself has a lower modulus than the metallic actuator assembly thereby reducing actuator stiffness.
Another inherent problem in use of any type of thermoplastic resin used to join a voice coil and actuator is that thermoplastic resin will shrink in size as the molten plastic cools after molding. In most circumstances, the over-molded resin has a tendency to shrink in a direction away from the voice coil thereby creating voids or thinned sections of thermoplastic resin around the voice coil. As well understood by those skilled in the art, thermoplastic resin itself does not have good adhesion characteristics with respect to adhering to the actuator; rather, the over-molded resin acts as a mechanical connection for joining the voice coil to the actuator. Therefore, gaps or voids in the thermoplastic resin due to shrinkage reduce the ability of the thermoplastic resin to provide a stiff connection.
FIG. 1 illustrates one prior art approach in securing a voice coil to the actuator. The method shown in this figure is described in the U.S. Pat. No. 5,122,703 and is illustrated at FIG. 6(A) thereof. In this approach, the actuator arm 100 has a single dovetail 102 located behind the bearing of the actuator and an over-molded thermoplastic resin 106 circumferentially encapsulates the outside diameter of the voice coil 104 with remaining portions of the over-molded resin 106 forming a border that encapsulates the dovetail 102. The sole joint created by the lone dovetail behind the bearing of the actuator is not a robust design and allows the voice coil and over-molded resin to displace from the dovetail 102 in micro-movements about the joint thereby resulting in a loss of stiffness. The relatively thin border of plastic resin along the outside diameter of the voice coil does not provide the required structural support to ensure the voice coil is sufficiently integrated into the actuator.
FIG. 2 illustrates another prior art approach wherein actuator 110 includes two integral yoke arms 112 which extend away from the proximal or rear portion of the actuator body. An over-molded resin 116 then encapsulates both of the yoke arms as well as the voice coil 114. The integral yoke arms increase overall stiffness as compared to the prior art approach of FIG. 1; however, by totally encapsulating the yoke arms, the thermoplastic resin shrinks in all directions towards the yoke arms. Accordingly, the resin is drawn away from the outside diameter of the coil creating gaps between the plastic and the outside diameter of the coil. These gaps reduce the overall actuator stiffness
FIG. 3 illustrates another example of a prior art approach in which an actuator 120 includes a pair of yoke arms 122, and a voice coil 124 is simply glued to the corresponding inner surfaces of the yoke arms. While the glue may provide good contact between the yoke arms and the coil, the glue may become soft at upper ranges of disk drive operating temperatures. The softening glue reduces stiffness of the actuator by allowing micro movements of the voice coil.
Another prior art approach as shown in FIG. 4 is to provide an actuator 128 with a plurality of teeth or curved protrusions on the inner surfaces of the yoke arms 130. These protrusions break up the normally smooth inner surface of the yoke arms thereby providing a greater surface area for the thermoplastic resin (not shown) to attach. However, even with the curved protrusions/teeth, a problem still exists in shrinkage of the plastic resin in a direction away from the yoke arms thereby reducing overall stiffness of the actuator.
Yet another prior art approach for attachment of a voice coil to an actuator is shown in the actuator 136 of FIG. 5. This approach requires the drilling of holes 140 in the yoke arms 138. The thermoplastic resin (not shown) fills the holes 140 thereby providing a locking feature to secure the voice coil (not shown). One distinct disadvantage in this approach is that the holes drilled in the yoke arms require a separate drilling operation which thereby increases the complexity of manufacturing, as well as potentially introducing contamination in a disk drive by small burrs or debris which may not be properly cleaned from the actuator prior to assembly of the disk drive.
One object of the present invention is to provide positive locking or positive retention features on the back end structure of an actuator which allows attachment of the voice coil without sacrificing or degrading stiffness of the actuator assembly. Another object of the present invention is to provide an actuator which may incorporate positive locking features at a minimum cost. Yet another object of the present invention is to provide a means to attach the voice coil by thermoplastic resin which takes advantage of the resin's natural tendency to shrink upon cooling.